Electrodes for arc furnaces are subjected to strong thermal and mechanical loads. The strong thermal loads result from the high working temperatures used in such arc furnaces, especially during the manufacture of electrosteel. Great mechanical loads arise during the running-in of electrodes due to contact with scrap and to the scrap parts falling into the smelt(so-called scrap dislocation). In addition the electrodes are caused to oscillate by electromagnetism, and the oscillations can attain substantial frequencies and amplitudes. Thus great acceleration forces arise which effect the electrodes as flexing or torsional loads. Moreover the generally rough and dust-laden operations during steel manufacture are an additional factor. Because of these conditions the connection of the shaft with the active portion of such electrodes raises considerable difficulties. Even so it is important that the connection between the shaft and the active portion should be simple in design, easy to detach and should cause only minor electrical inefficiencies or losses.
In the past screw connections between the shaft and the active portion were primarily favored (cf.e.g. DE-AS No. 27 39 483, out of the voluminous prior art). With this type of connection the shaft has a sleeve or the like at its lower end, which possesses an internal thread. At the upper end of the active portion there is a blind end bore which also has an internal thread. A screw-nipple is screwed into these two inner threads, which preferably consists of the same material as the active portion, i.e. primarily of graphite.
Special threads have been developed for such screw connections. These threads are not only adapted to the material of the active portion or of the screw nipple, but are also intended to take account to a large extent the operating conditions described above. The thread must for this purpose be as far as possible self-locking. It must also form good electrical contact surfaces, since at least sometimes a not insubstantial part of the electrical current flows via the screw nipple. In addition, tables have been compiled which show what torque must be applied in individual cases to the screw nipples in order to bring the contact surfaces between the shaft and the active portion into the desired pressure position which ensures an adequate electrical contact between said contact surfaces.
Certainly the screw solution has proven itself in use per se. But for many applications the changing of the active parts is a lengthy and costly process. In this connection designs would be desirable which make possible, while providing adequate thermal and mechanical strength, more rapid detachment of the active portion after its consumption from the relevant shaft and/or a faster and simpler mounting of an unused active portion on the shaft. Moreover the increasing cost of the active portions due to the rise in raw materials and energy costs compels the user to make the fullest use of the material in the active portion.
An electrode of the type prescribed in the preamble of patent claim 1 is already known(DE-OS No. 28 11 877) which allows in principle the simple detachment of a used active portion from the upper shaft and the mounting of an unused active portion on the shaft again. This known design is characterized in that the current transfer between the metal shaft and the active part and the detachable connection between the shaft and the active portion are functionally separated. However the securing device of the known electrode presupposes a special design of the upper end of the active portion. The upper end of the active portion is in fact equipped with a specially designed connector piece which consists of a round plate, on the under side of which an axial collar corresponding to the plate diameter is located, while on the upper side there is an extension of lesser diameter, which has a radially projecting flange. In a central borehole of the connector piece, a tension screw is provided to brace the connector piece with the active portion. For this purpose the upper part of the active portion is so designed that it embraces the head of the tension screw and engages in the collar which is conically shaped at the point of contact. Thereby the fracture of the upper end of the active portion under the influence of transverse forces and of the tension screw is prevented. On the side of the shaft, the securing device comprises a cage in the form of a hollow cylinder, which is equipped at its bottom end on the periphery with a plurality of recesses, into which clamping bodies are inserted. These bodies are radially movable and have the form of balls or rollers. The cage is linked by a piston to a hydraulic cylinder, and this piston can move the cage and with it the clamping bodies in relation to the cylinder in the axial direction. The clamping bodies then interact with an inclined control edge so that the clamping bodies, when raised by the hydraulic cylinder, are moved radially inward by said control edge, whereby they are positioned under an edge of an extension of the connector piece. This causes a positive locking of the active portion with the shaft.
The securing device of the known electrode just described is extremely complicated. This results primarily from the need to equip the active portion with a specially designed connector piece which has to be braced by a tension screw to the upper end of the active portion. This design is necessary because in view of the arrangement chosen, the material of the active portion is tension loaded. The tensile strength of the relevant materials for the active portions, especially of graphite, is however substantially less than the compression strength of the materials concerned. The arrangement chosen for the known solution using a connector piece and a tension screw for the active portion obviously makes the electrode more costly.
A further disadvantage of this system is the necessity to use metallic parts as securing elements which are not cooled in the hot active portion of the electrode.
In a substantially similar known electrode, instead of the ball mechanism just described, a tongs mechanism is used (U.S. Pat. No. 3,311,693, FIG. 2). In this design as well, the top end of the active part has to be equipped with a specially designed connector piece, so that the same disadvantages apply to this arrangement as in the case of the electrode design already described.